1. Field of the Invention
This invention generally relates to an aberration measuring apparatus and method, and particularly to an aberration measuring apparatus for measuring the wavefront aberration of an optical system used for a soft X-ray.
2. Description of the Related Art
When manufacturing a minute semiconductor element such as a semiconductor memory or a logic circuit by the use of photolithography technique, use has heretofore been made of a reduction projection optical system for projecting a circuit pattern depicted on a reticle or a mask (herein these terms are exchangeably used) onto a wafer or the like having a photosensitive agent applied thereto by a projection optical system to thereby transfer the circuit pattern.
A maximum dimension (resolution) which can be transferred by a reduction projection exposing apparatus is proportional to the wavelength of light used for exposure, and is inversely proportional to the numerical aperture (NA) of the projection optical system. Accordingly, the shorter is made the wavelength, the higher becomes the resolution. Therefore, the shortening of the wavelength of exposure light has been progressed with the demand for the minuteness of semiconductor elements in recent years, and as light sources, use is made of a super-high pressure mercury lamp (i-line (a wavelength of about 365 nm)), a KrF excimer laser (a wavelength of about 248 nm), an ArF excimer laser (a wavelength of about 193 nm) and a light source which can supply ultraviolet light of a shorter wavelength.
Semiconductor elements, however, have rapidly become minute, and lithography using ultraviolet light has limitations. So, in order to transfer a very minute circuit pattern of 0.1 μm or less, there has been developed a reduction projection optical system using a soft X-ray (extreme-ultraviolet light: EUV light) having a wavelength of the order of 5 nm to 15 nm which is still shorter in wavelength than the ultraviolet light.
In the wavelength area of the EUV light, the absorption of light by a substance becomes very great and therefore, such a refraction type optical system utilizing the refraction of light as is used in visible light or ultraviolet light is not practical, and in an exposing apparatus using the EUV light, use is made of a reflection type optical system utilizing the reflection of light.
As a reflection type optical element constituting the exposing apparatus using the EUV light, use is made of a multi-layer film mirror comprising two kinds of substances differing in optical constant from each other alternately laminated. For example, several tens of molybdenum (Mo) layers and silicon (Si) layers are alternately laminated on the surface of a glass substrate polished into a precise shape.
To evaluate the wavefront aberration of an optical system constituted by such a multi-layer film mirror, it is desirable to use a light having a wavelength in the vicinity of 13 nm which is an exposure wavelength, and as a light source having high brightness which can constitute an interferometer in such a wavelength range, there is a combination of synchrotron orbital radiation (SOR) and an undulator (see, for example, the following patent publications). These methods are shown, for example, in:                Japanese Patent Application Laid-Open No. 2000-097666        (Corresponding U.S. Pat. No. 6,312,373B and U.S. Pat. No. 6,456,382B);        Japanese Patent Application Laid-Open No. 2002-202449;        Japanese Patent Application Laid-Open No. 2002-258131        (Corresponding WO A1 2002054459);        Japanese Patent Application Laid-Open No. 2002-319539        (Corresponding U.S. AA 2002159040);        Japanese Patent No. 2002-324752        (Corresponding U.S. AA 2002159040);        Japanese Patent No. 2002-334831        (Corresponding U.S. AA 2002145717); and        Japanese Patent No. 2003-045794.        
However, the light source comprising a combination of SOR and an undulator is very expensive and large-scale and therefore, the measurement of wavefront aberration using such a light source is disadvantageous and is not realistic in terms of apparatus making and cost.
Accordingly, it is difficult to effect the evaluation of an optical system constituted by the multi-layer film mirror by the use of a light having a wavelength in the vicinity of 13 nm which is the exposure wavelength.